A magnetoresistive random access memory device (MRAM), which is one of non-volatile memory devices, includes a plurality of magnetic memory cells employing the giant magnetoresistive (GMR) effect. It is known that the GMR effect appears in multi-layer films which are alternately stacked by magnetic layers and non-magnetic layers. Magnetic resistance over a magnetic memory cell indicates minimum and maximum values when magnetic vectors in magnetic layers point in a same direction and in an opposite direction, respectively. The same and opposite directions of magnetic vectors are called "Parallel" and "Antiparallel," respectively. When GMR material is employed for a memory device, parallel and antiparallel directions, for example, are logically defined as a "0" state and a "1" state, respectively.
The MRAM device normally arranges magnetic memory cells on intersections of word and sense lines. The MRAM circuit, for example, is described in U.S. Pat. No. 5,699,293 entitled "A METHOD OF OPERATING A RANDOM ACCESS MEMORY DEVICE HAVING A PLURALITY OF PAIRS OF MEMORY CELLS AND THE MEMORY DEVICE", issued Dec. 16, 1997 assigned to the same assignee.
Activation of a word line and a sense line enables read and write states in the memory cell. The sense line is directly coupled to the memory cells in series and a sense current flows in the magnetic layers so that a sense current is affected by magnetic vectors in the magnetic layers and the current value is alternated according to the direction of magnetic vectors. Sensing the changes in the sense current value allows one to detect states stored in the memory cells. On the other hand, a writing process is carried out by applying a sufficient magnetic field to switch magnetic vectors in the magnetic layers. The word and sense currents both create a total magnetic field and apply it to the memory cell which stores states in the memory cell in accordance with directions of the word current.
The prior magnetic memory device reads and writes a single bit in a memory cell after activating a word line and a sense line. As a processor improves its speed, a memory device is also needed for high speed operation. To increase the operating speed, the applicant proposed the method of sequentially selecting a word line in U.S. Pat. No. 5,748,519 entitled "A METHOD OF SELECTING A MEMORY CELL IN A MAGNETIC RANDOM ACCESS MEMORY DEVICE", issued May 5, 1998 assigned to the same assignee and incorporated by reference herein. This invention teaches to sequentially select a word line after activating a sense line so that it saves time for the sense line selection and improves the cycle time. However it still handles a single bit information for reading and writing operations.
Accordingly, it is a purpose of the present invention to provide an improved non-volatile MRAM device having a high-speed operation.
It is another purpose of the present invention to provide an improved non-volatile MRAM device having a high density memory cell arrangement.